This invention relates to a manufacturing method of a graphene substrate and a graphene substrate manufactured by this method. The invention particularly relates to a manufacturing method of a graphene substrate, and a graphene substrate applicable to next-generation electronics, opto-electronics, and spintronics due to its exceptional electronic and optical properties, and excellent mechanical and chemical properties.
The information-oriented society now we see is supported by semiconductor devices represented by silicon-based CMOS (Complementary Metal Oxide Semiconductor) devices. Silicon semiconductor industries have heretofore achieved miniaturization by continuously reducing the size range to which microprocessing techniques such as lithography, etching, and deposition techniques are applied, from micrometers to several tens of nanometers, and thus have realized both high-scale integration and high performance. However, in recent years, thicknesses of silicon layers serving as semiconductor channels have become as small as the level of atomic layers. It is now pointed out that silicon is facing its limit as a material and its physical limit.
Graphene, which is an ultimately thin two-dimensional atomic layer film but is chemically and thermodynamically stable, is a novel semiconductor material possessing high potential to meet the requirements described above. Utilization of excellent properties of graphene may bring a possibility to realize new devices having superior performance exceeding that of existing devices.
Graphene is one single layer taken from graphite which is a layered material made only of sp2 hybrid carbon. As mentioned in the above, graphene is a single-atomic-layer planar material which is very robust to chemical reactions and stable thermodynamically. The graphene comprises a structure of a honeycomb like (pseudo) two-dimensional sheet in which regular six-membered rings with carbon atoms at the apexes are tightly arranged with no space therebetween. The carbon-to-carbon distance is about 1.42 angstroms (0.142 nm), and the thickness of the layer is 3.3 to 3.4 angstroms (0.33 to 0.34 nm) when it is on an underlying layer of graphite, whereas is about 10 angstroms (1 nm) when it is on any other substrate.
A plane of graphene may assume various sizes. A length on one side may vary from a molecular size of nanometer order up to theoretically infinity. While graphene in general comprises a single layer structure, while it often comprises two or more layers. Graphenes of a single layer, two layers, and three layers are called monolayer graphene, bilayer graphene, and trilayer graphene, respectively, and graphenes of up to about ten layers are collectively called few-layer graphene. All the graphenes except single-layer graphene are called multilayer graphene.
The electronic state of graphene can be described by a Dirac equation in its low-energy region as described in Ando, “The electronic properties of graphene and carbon nanotubus”, NPG Asia Materials 1(1), 2009, p 17-21 (Non-Patent Document 1). In this regard, graphene is in marked contrast with other materials than graphene, which are described well by Schrodinger equations.
Electronic energy of graphene has a linear dispersion relation with wavenumber in the vicinity of K point. More specifically, electronic energy of graphene can be represented by two straight lines with positive and negative gradients corresponding to a conduction band and a valence band. The point where these lines intersect is called Dirac point, at which graphene exhibits peculiar electronic properties that electrons of graphene behave as fermions with an effective mass of zero. Due to this, graphene has a mobility of 107 cm2V−1s−1 or more, that is the highest of all the existing materials, and yet has low dependence on temperature.
Single-layer graphene is basically a metal or semimetal with a bandgap of zero. However, when the size becomes nanometer order, the bandgap is generated, and a semiconductor is produced having a finite bandgap depending on the width and edge structure of the graphene. Bilayer graphene exhibits a bandgap of zero when there is no perturbation, whereas when such perturbation as to break mirror symmetry between the two graphene layers, for example, an electric field is applied to the graphene, it exhibits a finite gap according to the magnitude of the electric field.
For example, a gap of about 0.25 eV is generated in an electric field of 3V nm−1. Trilayer graphene exhibits metallic electronic properties in which a conduction band and a valence band overlap with each other by a width of about 30 meV. Thus, the trilayer graphene is close to bulk graphite in terms of the fact that the conduction band and the valence band overlap with each other. Graphene of four or more layers also exhibits metallic properties, and the properties gradually approach to electronic properties of bulk graphite as the number of layers increases.
Graphene is excellent in mechanical properties as well, and the Young's modulus of one layer of graphene is as high as 2 TPa (terapascals). The tensile strength of graphene is the highest of all the existing materials.
In addition, graphene has peculiar optical properties. For example, in a wide electromagnetic wave range from ultraviolet range (wavelength of 200 nm or less) to terahertz near range (wavelength of 300 μm or less), the transmissivity of graphene is represented by 1-nα (where n denotes a number of layers of graphene, that is equal to about 1 to 10, α denotes a fine-structure constant, that is represented by α=e2/2hc∈0=0.0229253012, where e denotes an elementary electric charge, h denotes a Planck's constant, and c denotes a dielectric constant of vacuum). Thus, graphene is represented only by fundamental physical constants, but not by material constant of graphene. This is the feature peculiar to graphene that is not found in any other materials.
Further, the transmissivity and reflectivity of graphene exhibit dependence on carrier density in terahertz spectral region. This means that the optical properties of graphene can be controlled based on electric field effect. It is known that other two-dimensional atomic layer thin films also have peculiar properties based on dimensionality.
Having exceptional electronic and optical properties and excellent mechanical and chemical properties as described above, graphene is expected to be used in a variety of industries from chemical to electronic industries. The fields of application of graphene are being expanded in various countries in the world, and graphene is being used for semiconductor devices and micro mechanical devices in the fields of next-generation electronics, spintronics, opto-electronics, micro/nanomechanics, and bioelectronics. For other two-dimensional atomic layer thin films as well as graphene, researches and developments are actively carried out for the purpose of industrially utilizing them.
When graphene is used for a channel of a field effect transistor (FET) or any other semiconductor device, a substrate for supporting the graphene (graphene substrate) is required.
A graphene substrate has conventionally been manufactured by the following four manufacturing methods:
(1) manufacturing method by peeling (see, for example, K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, “Two-dimensional atomic crystals”, PNAS, 102(30), 2005, p 10451-10453 (Non-Patent Document 2));
(2) manufacturing method by CVD (Chemical Vapor Deposition), (see, for example, Xuesong Li, Weiwei Cai, Jinho An, Seyoung Kim, Junghyo Nah, Dongxing Yang, Richard Piner, Aruna Velamakanni, Inhwa Jung, Emanuel Tutuc, Sanjay K. Banerjee, Luigi Colombo, and Rodney S. Ruoff, “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils”, Science, Vol. 324, 2009, p 1312-1314 (Non-Patent Document 3));
(3) manufacturing method by thermal decomposition of silicon carbide (SiC) (see, for example, M. Kusunoki, T. Suzuki, T. Hirayama, N. Shibata, and K. Kaneko, “A formation mechanism of carbon nanotube films on SiC (0001)”, Applied Physics Letters, Vol. 77 (4), 2000, p 531-533 (Non-Patent Document 4)); and
(4) manufacturing method by interface growth between gallium and amorphous carbon (see, for example, JP 2010-037128A (Patent Document 1) and Fujita, Ueki, Miyazawa, and Ichihashi, “Graphitization at interface between amorphous carbon and liquid gallium for fabricating large area graphene sheets”, Journal of Vacuum Science & Technology. Second series. B, vol. 77 (4), 2006, p 3063-3066 (Non-Patent Document 5)).
The manufacturing method by peeling of (1) is a method in which graphite and graphene pieces are peeled from graphite crystals such as natural graphite or highly oriented pyrolytic graphite (HOPG) with adhesive tape and pasted on a substrate.
The manufacturing method by CVD of (2) is a method in which a hydrocarbon such as methane is thermally decomposed or plasma decompose on a substrate comprising a metal catalyst segregated thereon or on a foil serving as a metal catalyst to grow graphene, and then the unnecessary metal catalyst is removed with acid or the like and the graphene is transferred onto another substrate.
The manufacturing method by thermal decomposition of silicon carbide (SiC) of (3) is a method in which graphene is grown by heat treating a SiC substrate at a high temperature (up to 1600° C.) to cause carbon atoms to aggregate on the substrate while evaporating silicon atoms from the surface of the substrate.
The manufacturing method by interface growth between gallium and amorphous carbon of (4) is a method in which a liquid gallium layer is brought into contact from above with an amorphous carbon layer segregated on a substrate at a high temperature (up to 1000° C.), so that graphene is grown on the amorphous carbon by interface reaction, and a composite layer consisting of a gallium layer, a graphene layer, and an amorphous carbon layer is transferred onto another substrate, and gallium is dissolved with acid to obtain a composite layer consisting of a graphene layer and an amorphous carbon layer. According to this method of (4), the layers are arranged, after one transfer process, in the sequence of the amorphous carbon layer, the graphene layer, and the substrate. In order to form graphene in the uppermost layer to meet the necessity in fabrication of the device, that is, in order to form the layers in the sequence of the graphene layer, the amorphous layer, and the substrate, one more transfer process, and hence two transfer processes in total must be performed.